The present invention relates to drills and more particularly to drill controls for automatically changing the speed of a bit in response to the hardness of a material being drilled.
For optimum drilling of a material, the speed of a drill bit must be matched to the hardness of the material. While this is relatively simple when a homogenous material is being worked, regulating the drill speed becomes difficult when the workpiece is a laminated material consisting of layers of material with different hardness. As the drill bit enters each layer of the laminate, its speed may have to be changed according to the relative hardness of the layer.
Industrial drills, such as the one shown in U.S. Pat. No. 4,648,756, provided an automatic feed of the drill bit into the material as it rotated. The operator merely placed a tubular drill nose against the surface of the workpiece and activated the drill. Internal gearing of the drill provided a positive translational feed of the drill bit an incremental distance into the workpiece for each rotation of the bit. The particular drill shown in this Patent also provided a mechanical mechanism for automatically shifting its speed to match the hardness of the particular layer of the laminate being drilled. The speed shifting was based on the depth of the drill into the workpiece as indicated by a cam block machined according to the thickness of the laminate layers and their relative hardnesses. A different cam had to be prepared for each type of laminated material.
Other drills, such as the one shown in U.S. Pat. No. 4,198,180, took advantage of the relationship between the thrust exerted by the drill and the hardness of the material to regulate the speed of the b&t. These types of devices had mechanical mechanisms which tripped at a preset thrust threshold to shift the drill to a higher or lower speed. The thrust threshold at which the drill shifted speeds could not easily be changed when different types of laminated materials were to be drilled. Furthermore, as the drill bit became dull, greater thrust had to be exerted in order for it to drill through each layer of the laminate. The prior mechanically shifted drills did not provide a mechanism for adjusting the shift thrust threshold to compensate for bit wear. In addition, these previous devices did not provide any mechanism for indicating to the operator when excessive bit wear occurred which necessitated changing or sharpening the bit.